This invention relates to a testing system and more particularly to an aqueous system for testing magnetizable articles.
It is always desirable to test an article before it is placed in use. Such testing can assist in determination of flaws not readily visible to the human eye. It is highly critical to determine the flaws. These otherwise invisible flaws can result in major weaknesses and a failure in the article with disasterous results. Thus, it is desirous to test articles.
One way to test an article is to completely destroy it and examine the resulting fractured pieces. This method is, however, destructive and denies the use of the part after substantial work is done to make the part. This testing is also based on statistical foundations. This method can only be used with a group of the articles, wherein one article from that group is selected at random for testing.
If the selected article passes the test, from a statistical standpoint, the odds as to whether the other articles are safe can be determined. Clearly the random selection of a part or an article for testing leaves too much to chance. There is no test of all the articles, and no positive indication of the safety of all the articles.
Furthermore, the destructive nature of the test renders the article unsuitable for use. Destructive testing is, therefore, costly and wasteful. Cost and waste are clearly factors to be reduced in the interest of improving production.
To avoid the problems caused by destructive testing, various methods are known. Typical methods of testing can be used depending on the nature of the article. For example, if an article is magnetizable, a certain test may be used to determine flaws in the article. The article--usually of steel or iron--is coated with a liquid bath containing magnetic inspection particles. At a suitable point during the inspection process, the article is magnetized. The particles are then attracted to variations in the flux field of the magnetized article. The variations are caused by the defects in the article.
In theory and in actual practice, the slurry containing the magnetic inspection particles coats the surface of the article being tested. The particles are then attracted to the defects in the article in the same manner as magnetic items are attracted to the poles of a magnet. The build-up of particles at the flaws caused by this magnetic force make the flaws clearly evident. In this fashion, flaws, otherwise invisible to the human eye, can be detected when placed under a proper light. This procedure is generally referred to as the wet method of magnetic particle inspection.
There are many key factors in applying the magnetic inspection particles to the article to be tested. The liquid medium for carrying the particles to the surface of the article to be tested must have appropriate viscosity, which allows for complete wetting of the surface of the article without inhibiting particle migration. This works in combination with the appropriate surface tension and corrosion inhibition level to allow for proper particle suspension and dispersion in the slurry--as well as providing the required protection from corrosion. If these factors are met for the liquid medium, better migration of the particles to the defect during and following the magnetizing process on the article is achieved.
The value of using a non-fluorescent medium to eliminate background fluorescence is considered, due to its importance to most applications. An aqueous medium meeting this requirement of eliminating background fluorescence can then be used with fluorescent magnetic inspection particles--provided the other criteria for magnetic particle inspection systems can be met.
To accomplish this purpose of magnetic particle inspection, a light industrial oil known in the industry as an inspection oil can be used. This oil has an appropriate viscosity and the ability to wet the magnetizable part thoroughly. The inspection oil allows for better migration of the particles in the carrier to the defect. It also acts to wet out and disperse the magnetic inspection particles. Furthermore, oil acts as a rust inhibitor, which is very important on all machined and finished parts--especially those parts made of iron or steel.
Yet oil being used as the liquid carrier also has a number of disadvantages. First, the oil is combustible or flammable. If the oil does burn, a great health hazard is created. The mere use of such a combustible ingredient consitutes a fire hazard to both life and property.
For the testing of critical machined parts such as those found in automotive and aircraft components, an oil medium is known to be useful due to the fact that an oily film provides some corrosion protection for the parts. However, one of the problems in using oil is, that in the case of pressing railroad wheels onto the highly machined axles, the oil tends to trap abrasive dust which interferred with the pressing operation.
Much more is known about oil as the carrier medium. This information does not take into account the importance of viscosity to surface coverage and particle migration. Viscosity has a great effect on magnetic inspection particles.
Furthermore, an individual working with the oil can suffer from dermatitis and exposure to fumes. These problems are further compounded by hazards inherent in the disposing of the used oil in an environmentally safe manner after such use. It is clear that such disposal problems complicate the use of an oil-based magnetic particle inspection system.
With all these problems found in the use of oil, it thus becomes desirable to find another appropriate liquid medium. One liquid lacking the fire hazard, dermatitis, fume and disposal problems is water. However, water can cause oxidation on the surface of the article being tested. Furthermore, water does not properly coat the surface of the article, nor does it permit proper dispersion of the magnetic inspection particles.
Wetting agents reduce the surface tension of water allowing it to wet out the magnetic inspection particles--as well as flow over and wet out the surface of the part being tested. They control the pH usually by the addition of borax and discourage corrosion by incorporating sodium nitrite.
Use of sodium nitrite has been regulated by United States Environmental Protection Agency. So other corrosion preventative mixtures are now in use as substitutes therefor. These substitute materials do not afford surface corrosion protection of the of the ferrous surface.
If environmentally safe components can be added to water to improve wettability and corrosion resistance, if is possible for water to be used as the carrying liquid for magnetic inspection particles. Its use as a liquid carrier can be further enhanced, if the viscosity can be adjusted to achieve the desired results in the migration of the magnetic inspection particles.
It thus becomes clear that it is highly desirable to develop a conditioned, water-based carrier to be used as an inspection medium in the wet method of magnetic particle inspection.